A servo system requires feedback information in order to close the servo loop. A servo command to the servo system to move or rotate a load to a new position will typically require the servo to generate a signal (an error signal) that is representative of an error between a present position of the load and the new commanded position. The error signal can then be used as a basis to move the load with an actuator to the new position. As the load approaches the new position, the error signal will tend to represent a smaller error value, indicating that the load is coming nearer to the new position.
A position sensor can be used to translate information related to the position of the load and/or the actuator into electrical information that can be provided to the servo system. The electrical information (i.e., the error signal) can be used in the servo system to make necessary adjustments to the servo commands used to control the actuator. It is desired that the position sensor have a linear response so that there can be optimum performance throughout the full range of motion of the servo.
A wide variety of sensor types has been used in the past as position sensors. Potentiometers have been used as position sensors in servo systems. The potentiometer uses electrical contacts to provide a voltage that can vary depending upon servo position and has a linear response. Furthermore, potentiometers can be inexpensive, permitting the construction of low-cost servo systems. Electromagnetic sensors, such as those using Hall Effect sensors, have been used as position sensors in servo systems. The electromagnetic position sensors have an advantage in that there are few mechanical parts to wear. Furthermore, since there is no friction involved in the position sensing, the precision of the position sensor is high. Optical sensors with single photo-sensors have also been used in servo systems. The optical position sensors have an advantage of few mechanical parts, no friction, long life, and so forth, as well as having consistent, predictable performance over a large number of position sensors.
One disadvantage of the prior art is that potentiometers make use of a mechanical mechanism to translate the positional information into electrical information. A mechanical mechanism will have friction since physical contact is required, which can lead to erratic performance, especially when the servo is starting or stopping. Furthermore, mechanical mechanisms can wear down with use; therefore, they can have a limited useful lifespan.
A second disadvantage of the prior art is that the electromagnetic sensors making use of Hall Effect sensors can have unpredictable performance across different position sensors and can require tweaking to provide consistent and predictable performance. The additional tweaking can increase the cost involved in the use of the electromagnetic sensors.
A third disadvantage of the prior art is that the optical sensors with single photo-sensors have had non-linear response. Therefore, the performance of the servo system can be sub-optimal throughout the full range of motion of the servo.